Known techniques for processing an audio signal derived from string instruments are limited by the difficulty of providing an accurate triggering signal to enable event-driven signal processing techniques. Accordingly, most signal processing techniques currently used in real-time with string instruments are continuous in the sense that a signal processing process is not stopped and started on an event basis. Typical audio effect processes such as echo, reverberation, phasing, panning, chorus and flanging are usually continuous in nature since the effect is applied to the audio signal continuously for as long as the effect is desired.
An attempt to provide a triggering signal to enable more sophisticated signal processing is described in U.S. Pat. No. 4,235.144. This prior art document discloses a conductive pick connected to a contact sensor which senses conductive contact between the strings of the guitar and the conductive pick. In this arrangement, breaking contact between the pick and the string initiates a special musical effect.
It has been appreciated by the inventor of the present invention however that this prior art arrangement suffers numerous technical defects to the extent that it cannot be successfully employed to provide a triggering signal reliable enough to enable sophisticated event-driven signal processing. In particular, the inventor of the present invention has discovered that the conductive contact between the string and the prior art conductive pick can be subject to numerous imperfections leading to false triggering. This can be exacerbated by the habit of some string instrument players of resting their pick on the string before actually plucking the string. As the prior art arrangement triggers from the moment when conductive contact between the pick and the string is broken, the imperfect conductive connection can result in false triggering. Other factors leading to imperfect triggering by the prior art arrangement of U.S. Pat. No. 4,235,144 include: a string and/or the pick may be tamished, thereby inhibiting stable conductive contact; the pressure of the pick on the string may not be constant due to the player touching the pick against the string lightly; and larger gauge strings in particular can be vibrating quite vigorously towards and away from the pick, thereby initiating and breaking conductive contact prior to plucking of the string. Whilst this imperfect triggering may suffice for the relatively simple effects outlined in the abovementioned U.S. patent, it has been found by the inventor of the present application not to suffice for slightly more sophisticated triggering such as MIDI triggering, Control Voltage and Gate triggering, in other words, the type of triggering required for the signal processing provided by modern synthesizers.